1. Field of the Invention
This invention relates to a circuit that is used to detect pen faults in an ink jet printer. Specifically, the invention relates to a circuit that checks for open circuit and short circuit faults in the print driver circuitry.
2. Relevant Background
Ink jet printers are cost effective tools for producing high-quality color and black-and-white pictures and text. Conventional print mechanisms in ink jet printers include one or more ink-filled print cartridges coupled to a printer head. The printer head has a “pen” that includes one or more electrically controllable ink flow channels that couple to an ink reservoir and operate to emit the ink through a nozzle onto a print substrate (e.g., a piece of paper).
Thermal ink jet printers use heat to fire ink onto the paper. The ink is heated by a heating element associated with each nozzle. A power driver circuit supplies a pulse of current to each heating element at a predetermined time causing ink proximate to the heating element to vaporize and create a bubble until the pressure forces the bubble to burst and emit an ink drop in the order of 5-10 picoliter. The bubble then collapses as the heating element cools, and the resulting vacuum draws ink from a reservoir to replace the ink that was emitted.
Printing resolution is largely determined by the frequency with which the print head nozzle can be cycled through these steps. To heat the element rapidly, a significant amount of energy is delivered to the heating element in a short amount of time. For example, a power supply in the 10-24 volt range may be used to deliver the power pulse. Because this power supply voltage is higher than conventional logic power levels (e.g., 2.7-5.0 volts), the switches used to deliver power to the heater elements are high voltage devices.
Performance of the print head is significantly affected by the ability to consistently and reliably apply precisely determined quantity of power to the heating element. Unfortunately, contaminants in the ink and elsewhere can create short circuits (i.e., excessive leakage current) and open circuits (i.e., excessive parasitic resistance) in the printer head that cause the nozzles to misfire or not fire at all. Excessive leakage and parasitic resistance causes power to be diverted from the heater element and therefore affect printing performance. In addition, faults can occur in the connection between the print cartridge and the cartridge driver circuitry that result in short and open circuits.
In light of these problems, many ink jet printers include fault check circuits that check the printing mechanism for short and open circuits that degrade the quality of printed images and text. These fault check circuits are implemented as integrated circuit devices that couple to the drive circuits of the pen. The fault check circuitry detects when an open or short circuit exists in the circuitry that drives an ink jet nozzle so that the faulty nozzle may be shut down, repaired, or replaced, among other corrective actions.
The fault check circuitry essentially applies a voltage or current signal to the nozzle units to measure the equivalent resistance (Req) of the nozzle unit. The equivalent resistance is compared to a threshold resistance (Rth) where Rth is established at a value that would indicate excessive leakage or parasitic resistance. In practice this fault check is performed by applying a test signal to the pen circuitry and measuring a responsive signal. Comparator logic is used to compare the response signal to a reference signal where the reference signal is based on (i.e., proportional to) the value of Rth. Hence, for the comparator logic to operate properly, the value of Rth must have a known relationship to the equivalent resistance Req of the pen.
Unfortunately, conventional fault check circuitry includes high voltage switches that are coupled to the pen circuitry so that the fault check circuitry is not damaged by the high voltages applied during normal operation. Consequently, the fault check mechanism has a relatively large die size associated with high voltage devices. Thus, there remains a need for pen fault check circuitry that has a reduced die size.
Another problem with conventional fault check circuitry is that false readings, both positive and negative, can be generated because Rth is determined by components integrated with the fault check circuitry and is not readily adjusted to match the pen circuitry. As a result, the Rth value is affected by a number of variables such as manufacturing variances, changes in environmental conditions, and operating conditions such as supply voltage variations. In conventional circuits the value of Rth is sensitive to changes in the power supply voltage, internal gain of the pen fault circuit logic devices, resistance variation of resistors within the pen fault circuit logic devices, operating temperature, and other variables. It would be desirable to compensate the comparator logic for these variations.
Still another problem with fault check circuitry is that it can be either too sensitive (or not sensitive enough) to faults in the printer circuitry. Unfortunately, the use of integrated components in the reference circuitry of the fault detection logic makes it difficult to adequately adjust sensitivity by manipulating component parameters (such as gain and resistance values) in the comparator logic. Hence, there remains a need for fault check circuitry with improved ability to adjust sensitivity.
These and other problems with conventional fault check circuitry for ink jet printers are addressed by the present invention.